ABSTRACT: Marine algae produce a variety of secondary metabolites that function as herbivore deterrents. Algal metabolites, however, often fail to deter damage by some herbivores such as mesograzers that both live and feed on their host alga. In addition,the degree to which intraspecific chemical variation in an alga affects a mesograzer¹s feeding behavior and its ability to deter predators is poorly understood. The red alga Portieria hornemannii contains the secondary metabolites apakaochtodene Aand B, which have been shown to vary in concentration among sites on Guam and act as significant deterrents to fish feeding. On Guam, the sea hare Aplysia parvula preferred and grew best when fed its algal host P. hornemannii. However, highconcentrations of P. hornemannii crude extract and the pure compounds apakaochtodene A and B acted as feeding deterrents to A. parvula. Despite differences among sites in the levels of apakaochtodenes A and B, A. parvula showed nosignificant preference for P. hornemannii from any one location. Aplysia parvula found on P. hornemannii sequestered apakaochtodenes, and both whole animals and body parts were unpalatable to reef fishes. Sea hares found on the redalga Acanthophora spicifera, which contains no unpalatable secondary metabolites, had no apakaochtodene compounds and were eaten by fishes. This observation is consistent with the hypothesis that diet-derived algal metabolites in sea hares play arole in deterring predation.